This invention relates generally to electrophotographic printing machines, and more particularly concerns such a machine including a post-fusing substrate moisturizing and decurling device.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document.
After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles on the copy sheet are heated to permanently affix the powder image to the copy sheet.
The foregoing generally describes a typical black and white electrophotographic printing machine. With the advent of multicolor electrophotography, it is desirable to use an architecture which comprises a plurality of image forming stations. One example of the plural image forming station architecture utilizes an image-on-image (IOI) system in which the photoreceptive member is recharged, re-imaged and developed for each color separation. This charging, imaging, developing and recharging, re-imaging and developing, all followed by transfer to paper, is done in a single revolution of the photoreceptor in so-called single pass machines, while multipass architectures form each color separation with a single charge, image and develop, with separate transfer operations for each color. The single pass architecture offers a potential for high throughput.
In order to fix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to apply pressure and elevate the temperature of the toner to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of the support medium (typically paper). Thereafter, as the toner material cools, solidification of the toner material occurs, causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fixing of electroscopic toner images onto a support has been to pass the support bearing the toner images between a pair of opposed roller members, at least one of which is internally heated. During operation of a fixing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls and thereby heated under pressure. A large quantity of heat is applied to the toner and the copy sheet bearing the toner image. This heat evaporates much of the moisture contained in the sheet. The quantity of heat applied to the front and back sides of the sheet are often not equal.
One problem associated with moisture loss in paper is paper curl or waviness. As sheets pass through the fixing system, moisture is driven out and the sheet temperature is elevated. If after fixing, the sheet is then allowed to rest in a collection area fully exposed to its ambient surroundings, its moisture content will equilibrate with the environment through absorption of moisture across the full face of at least one side of the paper sheet. If, however, the copy sheet becomes part of a large compiled set, both sides of all of the papers in the compilation (except for the top sheet) will effectively be sealed off from the moisture within the atmosphere. The only route available to this desiccated paper for moisture re-absorption is through the edges of the sheets, leaving the moisture content of the central portions of the sheets relatively unchanged. This uneven pattern of moisture re-absorption results in edge stresses that lead to paper curl or waviness along the edges of the paper. The resulting wave pattern may typically have an amplitude of 1/8 inch to 1/4 inch.
In addition to being cosmetically unsightly, the edge wave or curl creates a secondary handling problem, in that pages having such a wave pattern along their edges are more difficult to feed to secondary paper handling machines, such as a binder apparatus. For this reason, printers continue to favor the use of offset presses, for large compilations.
Conventional decurlers have relied primarily upon bending the copy sheet after fusing in order to counteract the effects of curl. Ordinarily, conventional decurlers do not take into account the moisture lost from the paper during the fusing process nor time and other factors that change after or post fusing.
A number of solutions to this problem have been advanced. One proposed solution is to use an offset press dampening system to add moisture to each sheet as it exits the copier. These systems typically rely on the generation of a pool of water at a roll interface to distribute the water evenly along the rolls. Such systems usually operate with a web paper supply and their use with a cut sheet feeder system creates some difficulties not previously contemplated or addressed. Normal dampening systems are more appropriate for use with conventional offset presses.
U.S. Pat. No. 4,375,327 attempts to solve the problem of wave curling as it arises due to another cause: adherence of paper to a roller fixing device and does not address the problem caused by moisture loss. U.S. Pat. No. 4,652,110 (the contents of which are hereby incorporated by reference) attempts to replenish moisture lost in the fixing process by collecting moisture as it is driven off the copy sheet for reapplication to the sheet at a later time.
There remains a need for a system for preventing the edge curl or waviness caused by the loss of moisture from the copy sheet during the fixing step of electrostatographic reproduction or printing, as well as by other changing factors after the fusing or fixing step.